This invention relates to the control of the power applied to a motor and more specifically to the variation of input power to an AC induction motor in function of the loading conditions.
An induction motor runs most efficiently when it is fully loaded, that is, when the counter-electromotive force developed between the fields and the armature is almost equal to the power supplied to the motor. When the load is reduced without reducing applied voltage or when the applied voltage is raised without increase in load, the current remains relatively high, causing internal heat losses. Typically, the no-load current of a single-phase motor can reach 90% of the rated load current. In three-phase motors the no-load current is approximately 50%-60% of the rated load current. The current lag between voltage and current tends to increase from 30.degree. at full load condition to 80.degree. in an unloaded motor. Large users of motors with cyclic loads are often penalized by the utility companies for their low power factor.
The most practical way to control the power factor of an induction motor which is subject to wide variations in load or line voltage is to reduce the voltage applied to the motor every time the phase lag between the voltage and current exceeds a predetermined, desirable value.